Support custom mpv.conf (#327)

Part of #235 
Closes #305

Adds a preference for specifying a custom `mpv.conf`

Also adds a preference to toggle between `vo=gpu` and `vo=gpu-next`, see
https://github.com/mpv-player/mpv/wiki/GPU-Next-vs-GPU

There are some caveats:
* Multi-line text entry via a remote is rough
* [Track selection
options](https://mpv.io/manual/stable/#track-selection) may not work as
expected because Wholphin manages this itself
* Bad settings will crash the app hard

Dev note: This rewrites the text field inputs to use the [state based
implementations](https://developer.android.com/develop/ui/compose/text/migrate-state-based)

app/src/main/java/com/github/damontecres/wholphin/preferences/AppPreference.kt

@@ -701,6 +701,24 @@ sealed interface AppPreference<Pref, T> {
                 summary = R.string.disable_if_crash,
             )
 
+        val MpvGpuNext =
+            AppSwitchPreference<AppPreferences>(
+                title = R.string.mpv_use_gpu_next,
+                defaultValue = true,
+                getter = { it.playbackPreferences.mpvOptions.useGpuNext },
+                setter = { prefs, value ->
+                    prefs.updateMpvOptions { useGpuNext = value }
+                },
+                summaryOn = R.string.enabled,
+                summaryOff = R.string.disabled,
+            )
+
+        val MpvConfFile =
+            AppClickablePreference<AppPreferences>(
+                title = R.string.mpv_conf,
+                summary = null,
+            )
+
         val DebugLogging =
             AppSwitchPreference<AppPreferences>(
                 title = R.string.verbose_logging,
@@ -851,7 +869,11 @@ val advancedPreferences =
                         ),
                         ConditionalPreferences(
                             { it.playbackPreferences.playerBackend == PlayerBackend.MPV },
-                            listOf(AppPreference.MpvHardwareDecoding),
+                            listOf(
+                                AppPreference.MpvHardwareDecoding,
+                                AppPreference.MpvGpuNext,
+                                AppPreference.MpvConfFile,
+                            ),
                         ),
                     ),
             ),

app/src/main/java/com/github/damontecres/wholphin/preferences/AppPreferencesSerializer.kt

@@ -68,6 +68,7 @@ class AppPreferencesSerializer
                                         .apply {
                                             enableHardwareDecoding =
                                                 AppPreference.MpvHardwareDecoding.defaultValue
+                                            useGpuNext = AppPreference.MpvGpuNext.defaultValue
                                         }.build()
                             }.build()
                     homePagePreferences =